1. Field of the Invention
The present invention relates to a method for manufacturing a display device.
2. Description of the Related Art
In recent years, flat panel displays (FPDs) typified by liquid crystal displays (LCDs) and electroluminescent (EL) displays have been attracting attention as display devices taking the place of the conventional CRTs. In particular, development of large-area liquid crystal television devices equipped with large-size active matrix liquid crystal panels has been a major issue for liquid crystal panel manufacturers. Moreover, development of large-area EL television devices has recently been carried out following to the development of the liquid crystal television devices.
A thin film transistor (hereinafter, also referred to as a TFT) using an amorphous silicon is used as a semiconductor element for driving each pixel in the conventional liquid crystal display device or EL display device (hereinafter, also referred to as a light emitting display device).
Meanwhile, the conventional liquid crystal television devices have defects of image deblurring caused by the limits of viewing angle characteristics and the limits of high-speed operation due to a liquid crystal material etc. Recently, an OCB (optically compensated bend) mode has been proposed as a new display mode for solving the above problem (see non-patent document 1).    Non-Patent Document 1: Yasuaki Nagahiro et al., Nikkei Microdevices' Flat Panel Display 2002 Yearbook, Nikkei BP Marketing, Inc., October 2001, pp. 102-109
However, when a TFT using an amorphous semiconductor film is driven by flowing direct current therethrough, the threshold value is easily varied, causing the variation in characteristics of the TFT easily. Therefore, the fluctuation of luminance is caused in a light emitting display device that uses this TFT using the amorphous semiconductor film in a switching pixel. This phenomenon is especially prominent in a large-area television device with 30 inches or more (typically, 40 inches or more) in diagonal, which results in a serious problem of the deterioration of image quality.
Furthermore, it has been thought that a switching element that can be operated at high speed is required in a liquid crystal display device employing the OCB mode or the like in order to improve the image quality of the LCD. However, the TFT using the amorphous semiconductor film has the limits of high speed operation. Therefore, it is difficult to realize a high-performance liquid crystal display device.